In order to determine the length of side CD, use the tangen of 60 degrees as follow:
[tex]\tan 60=\frac{18\sqrt[]{3}}{CD}[/tex]
Solve the previous expression for CD and simplify, as follow:
[tex]CD=\frac{18\sqrt[]{3}}{\tan 60}=\frac{18\sqrt[]{3}}{\sqrt[]{3}}=18[/tex]
Hence, the length of side CD is 18
